Electrical cables comprise at least one electrical conductor, e.g. a wire or a strand, as well as an insulating sheath made of plastic, which surrounds and electrically insulates the conductor along its length. In order to connect the cable to other electrical components (e.g. plugs, terminals), part of the insulating sheath is removed at one end of the cable and the conductor is exposed. This procedure of removing the insulating sheath is also referred to as stripping.
The stripping process nowadays usually takes place automatically by means of devices which comprise a stripping blade. Two stripping blades are preferably provided, which are displaceable towards the cable from opposite directions along a straight line, in order to sever the insulating sheath and to pull it off from the conductor with a movement of the stripping blades.
Ideally, the insulating sheath should be completely severed and pulled off in this stripping process without the stripping blades touching the conductor. Since the contacting of the conductor by the stripping blades is used however to signal the severing of the insulating sheath, a brief contact with the conductor in the region in which the stripping is to be carried out is however desired. In practice, therefore, it is important to avoid damage during this brief contact with the conductor.
There is therefore a conflict in the selection of the stripping parameters. If the incision depth is not selected large enough, in order to avoid damage to the conductor, the conductor is not expected to be contacted; if the insulating sheath is not sufficiently cut into, it may happen that the insulating sheath is torn off during the pulling-off in such a way that clean insulation edges do not arise. In addition, it is possible for the insulating sheath to slip away from the stripping blades and not be completely pulled off.
On the other hand, if the incision depth is selected too large, damage to the conductor results, which may possibly prevent the processed cable being used. In addition, stresses on the stripping blades arise.
To avoid this problem, it is proposed in WO2014/060218A1 to detect the contacting of the conductor by a stripping blade by means of a capacitive sensor and to readjust the stripping blade in the case of the presence of contacting. For this purpose, a reference capacitor is charged with a voltage and connected preferably periodically to the capacitance of the stripping blades, which increases if the stripping blade makes contact with the metallic conductor. Depending on whether the stripping blade makes contact or not with the conductor, a change in charge of the reference capacitors and a corresponding change in voltage results, which is compared with a reference value in order to establish whether contact is present or not.
In order to improve the stripping processes further, a method is proposed in EP 2 919 340 A1, by means of which the stripping parameters can be optimized. For this purpose, the incision depth at which the conductor is contacted is determined in an optional first phase. In a second phase, stripping takes place with severing and pulling-off of the insulating sheath. After the severing of the insulating sheath, the stripping blades are moved apart by a predetermined measure, the so-called “wayback”, which is selected such that the conductor is no longer contacted during the pulling-off of the severed insulating sheath, but the insulating sheath is securely held. In the second phase, cables are stripped sequentially until no further contacting of the conductor occurs during the severing of the insulating sheath on the one hand and the pulling-off of the insulating sheath on the other hand. In an optional third phase, the cable processed with the optimum stripping parameters is examined visually to check the results of the optimization.
It should be noted, however, that the cable geometry is often not known and is also not constant over the entire cable length. The cable geometry can change inside the same cable bundle from production batch to production batch, so that previously optimized stripping parameters are no longer satisfactory. The known determination of the stripping parameters also requires a plurality of incisions which have to be statistically evaluated, which is time-consuming and leads to rejection of material. The visual inspection of the conductor in phase 3 is again associated with a considerable outlay.
As mentioned, not all contacting of the conductor by the stripping blades is critical. In order to establish which of the stripped cables meets to the quality requirements, a method is proposed in EP 2 717 399 A1 for monitoring the stripping processes. After the severing of the insulating sheath, during the pulling-off process, the length positions of the stripping blade are determined at which it has been determined by means of a detection device that at least one of the stripping blades has touched the conductor. The evaluation of the length positions at which conductor contacts have taken place then permits a classification of the stripped cables.
The mentioned methods permit stripping parameters to be optimized from point to point and a quality analysis and classification of the processed cables to be made from point to point. The described processes for optimizing the operating parameters of the stripping device, however, are still very time-consuming.
It should further be noted that when the described contacting of the conductors takes place, not only are the conductors damaged, but the stripping blades are also stressed and subjected to greater wear.